CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart) are often used to ensure that information submitted to a computer system was submitted by a legitimate user rather than an automated system. A CAPTCHA is a type of challenge-response test used to ensure that an account request is not generated by an automated system, where the most common type of CAPTCHA requires the user to alphanumeric characters rendered as a distorted image that appears on a display screen. Assuming automated systems are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be a legitimate user.
In response to recent CAPTCHAs designs that more difficult for an automated system to solve, malicious users have turned to human-computation attacks utilizing “human CAPTCHA farms”. These “human CAPTCHA farms” are employed to solve the CAPTCHAs displayed on a webpage, while the automated system automatically fills in and submits the rest of the webpage. The economic model behind the use of the “human CAPTCHA farm” is to use a large number of low skilled workers to quickly solve separated CAPTCHAs that are in a queue. The incentive for using these “human CAPTCHA farms” can be reduced by disrupting the economic model that makes their use attractive. One approach is to reduce the throughput of these “farms” so that the economics behind the “human CAPTCHA farm” can be shifted to the point where it is no longer worth the time or money to solve CAPTCHAs.
It is in this context that embodiments of the invention arise.